Method of forming a passivation film

ABSTRACT

A reduced pressure device the reduced pressure chamber of which is constructed of stainless steel, and includes a passivation film formed on the exposed interior surface thereof. The film has a thickness of more than 50 Å and is composed of two or more layers. One layer contains mainly chrome oxide formed in the vicinity of the interface of the stainless steel and the passivation film. The other layer contains mainly iron oxide formed in the vicinity of the surface of the passivation film. A passivation film may also be used with a thickness of more than 50 Å and containing mainly a mixture of chrome oxide and iron oxide. Lastly a passivation film may also be used with thickness of more than 50 Å and containing mainly chrome oxide.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 08/097,861, filed Jul. 26, 1993 now abandoned, which is acontinuation of U.S. patent application Ser. No. 07/990,549, filed Dec.14, 1992 now abandoned, which is a continuation of U.S. patentapplication Ser. No. 07/536,547, filed Jul. 10, 1990 now abandoned,which is a continuation of International application PCT/JP89/00023,filed Jan. 11, 1989, which designated the United States; the presentapplication is also a continuation of U.S. patent application Ser. No.07/721,450, filed Aug. 8, 1991 now abandoned, which is a continuation ofInternational application PCT/JP89/01308, filed Dec. 26, 1989, whichdesignated the United States; and the present application is also acontinuation of U.S. patent application Ser. No. 08/121,174, filed Sep.13, 1993, now U.S. Pat. No. 5,313,982, which is a continuation of U.S.patent application Ser. No. 07/922,370, filed Jul. 29, 1992 nowabandoned, which is a continuation of U.S. patent application Serial No.07/465,226, filed Mar. 5, 1990 now abandoned, which is a continuation ofInternational patent application PCT/JP89/00690, filed Jul. 2, 1989,which designated the United States.

BACKGROUND OF THE INVENTION

The present invention relates to a reduced pressure device, and moreparticularly to a reduced pressure device capable of realizing anultrahigh vacuum and ultraclean process.

In recent years, techniques for realizing an ultrahigh vacuum ortechniques for supplying fixed gases into a vacuum chamber at a smallflow rate to provide an ultraclean, reduced pressure atmosphere havebecome very important. Such techniques are widely used in the study ofmaterial characteristics, the formation of various films, and themanufacture of semiconductor devices with advances in high vacuumtechniques. It is therefore desired to obtain a reduced pressureatmosphere, wherein the impurity particles and molecules are reduced tothe smallest limit.

In order to enhance the degree of integration of an integrated circuit,the ability to use semiconductor devices which are composed of unitelements, having sizes being 1 μm to submicron and even to 0.5 μm orless is intensively sought as the size of unit elements decreases yearby year.

To manufacture such semiconductor devices it is necessary to repeatedlycarry out film formation processes and etching processes for thoseformed films according to fixed circuit patterns. Usually, suchprocesses are performed by placing silicon wafers into a vacuum chamberwhich has been evacuated to an ultrahigh vacuum state or into a vacuumchamber at a reduced pressure atmosphere whereinto fixed gases areintroduced. If impurities appear in the vacuum chamber during theseprocesses, problems such as the deterioration of film quality and areduction of the precision of fine processing will take place. This isthe reason that an ultrahigh vacuum or an ultraclean reduced pressureatmosphere are required.

One of the most important reasons for hindering the realization of anultrahigh vacuum and an ultraclean atmosphere has been the release ofgases from the surface of stainless steel, which material is widely usedin chambers and piping systems. In particular, moisture adsorbed in thesurfaces which will be released at a vacuum or reduced pressureatmosphere acts as the greatest source of contaminations.

FIG. 8 shows, for a prior art devices the relationship of the total gasleakage amount of a system including a gas piping system and a reactionchamber (the total amount of gases released from the internal surfacesof the piping system and the reaction chamber and the gases due toleakage into the chamber from the exterior) and the contamination of gasat different flow rates.

In order to realize a highly precise process, there is a tendency toadopt smaller and smaller gas flow rates. For example, the selection ofa flow rate of 10 cc/min or less has become generally accepted. If theflow rate of 10 cc/min is used and the system total leakage is in arange of 10⁻³ ˜10⁻⁶ Torr l/sec for a currently used prior art devicesthe purity of gas will reduce to 10 ppm˜1%. Therefore such a process isfar from being an ultraclean process.

The present invention comprises an ultraclean gas supply system andpermits reducing the external leakage amount below the detectable limitof 1×10⁻¹¹ Torr l/sec of existing detectors. However, because ofinternal leakages i.e., the existence of released gas emanating from theinternal stainless steel surfaces, the present invention fails to lowerimpurity concentrations in a reduced pressure atmosphere In the case ofstainless steel, the minimum amount of gas released from surfaces whichare treated with prior art ultrahigh vacuum techniques is 1×10⁻¹¹ Torrl/sec·cm². By way of example, since the internal exposed surface area ofa chamber is at least 1 m², the total leakage amount will be 1×10⁻⁷ Torrl/sec and gas with a purity of 1 ppm can be obtained only if the gasflow rate is 10 cc/min. Therefore it is evident that, if the gas flowrate is decreased still further, the purity will drop further.

In order to lower the degassing components released from the internalsurface of the chamber to the same level of 1×10⁻¹¹ Torr l/sec·cm², asurface treating technique for reducing the amount of gas from stainlesssteel is required.

Many gases of all kinds are employed in the semiconductor manufacturingprocess such as relatively stable general gases (O₂, N₂, Ar, H₂, He) andother special gases with, respectively, strong reactivity, corrosivityand toxicity. The existence of moisture in a special gas atmosphere willcause a hydrolysis reaction which produces hydrochloric acid and fluoricacid, since BCl₃ and BF₃, etc., all having strong corrosivity, occur inspecial gases. Because of considerations of reactivity,corrosion-resistance, high strength, readiness of secondary processing,weldability and the ability to polish internal surfaces, stainless steelis usually used as the material for piping and chambers for dealing withthese gases.

However, although the corrosion-resistance of stainless steel in anatmosphere of dry gas is very good, stainless steel will be easilycorroded if it is placed in an atmosphere formed of chlorine or fluorinegases. Because of the above-mentioned facts it is necessary to treatstainless steel for corrosion-resistance after polishing the stainlesssteel surface. There have been several treatment methods such as Ni-W-Pcoating (method of cleanness coating) This method not only causescracks, and easily forms pin holes but also has the disadvantage thatthe amount of moisture and solvent residue absorbed on the internalsurfaces is great since the process adopts the method of wet coating.Moreover there are other methods such as passivation treatment forproducing a thin oxide film on the surface of metal. Stainless steel canbe passivated only by immersing it into a liquid containing an efficientoxidizer. The passivation treatment of stainless steel is usuallycarried out by immersing it into a solution of nitric acid. However,because this method is still a wet process there remains much residue ofmoisture and treatment solution in the pipes and chambers. Particularlychlorine and fluorine gases will cause severe damage to stainless steelbeing treated in this way.

Therefore although it is very important for an ultrahigh vacuumtechnique and semiconductor process to use chambers and gas supplysystems constructed of stainless steel on which a passivation film isformed for occluding and adsorbing less moisture and which is capable ofresisting damage caused by corrosive gases, such a technique has notbeen developed in the prior art.

In view of the drawbacks of the prior art techniques it is an object ofthe present invention to provide an ultrahigh vacuum and ultra-cleanreduced pressure device capable of reducing impurities resulting fromreleased gas and having excellent corrosion-resistance.

Another object of the invention is to provide a reduced pressure devicecapable of performing self-cleaning and self-maintenance.

SUMMARY OF THE INVENTION

The concerned reduced pressure device according to the presentinvention, in one form thereof, comprises a main body which isconstructed of stainless steel. A two or more layer passivation filmwith a thickness greater than 50 Å is formed on at least a part of theexposed interior surface of the stainless steel located inside thedevice. One layer contains mainly chromium oxides formed in the vicinityof the interface of the stainless steel and the passivation film. Theother layer contains mainly iron oxides formed in the vicinity of thesurface of the passivation film It is desirable that a film is thenformed by heating the stainless steel to allow it to be oxidized at atemperature between 150˜400° C.

The reduced pressure device according to the present invention in oneform thereof, comprises a main body which is constructed of stainlesssteel A passivation film with a thickness greater than 50 Å andcontaining mainly a mixture of chromium oxides and iron oxides is formedon at least a part of the exposed interior surface of stainless steellocated inside the device.

It is desirable, as in the case of the passivation film that a film witha thickness of more than 100 Å is then formed by heating the stainlesssteel to allow it to be oxidized at a temperature between 400˜500° C.

The reduced pressure device according to the present invention, in yetanother form thereof, comprises a reduced pressure device the main bodyof which is constructed of stainless steel A passivation film with athickness greater than 50 Å and containing mainly chromium oxides isformed on at least a part of the exposed interior surface of thestainless steel located inside the device.

It is desirable, as in the case of the passivation film that a film witha thickness of more than 130 Å is then formed by heating the stainlesssteel continuously for nine (9) hours to allow it to be oxidized at atemperature greater than 550° C.

The reduced pressure device according to the present invention, in yet afurther form thereof, is characterized in that the stainless steel onwhich the passivation film is formed has a surface which possesses aflatness with a height difference, between convex parts and concaveparts within any area 5 μm in radius, of less than 1 μm.

The reduced pressure device according to the present invention, in stillanother form thereof is characterized in that a gas supply unit forsupplying ultrahigh pure gases is connected to the reduced pressurechamber. The main body of the gas supply unit is constructed ofstainless steel, and the passivation film is also formed on at least apart of the exposed interior surface of stainless steel inside the gassupply unit.

The reduced pressure device according to the present invention, in yetstill another form thereof is characterized in that a gas cylinder forsupplying ultrahigh purity gases via the gas supply unit is connected tothe reduced pressure chamber. The main body of the gas cylinder isconstructed of stainless steel. The passivation film is also formed onat least a part of the exposed interior surface of the stainless steelinside the cylinder.

The reduced pressure device according to the present invention, in astill further embodiment thereof, is characterized in that, in theembodiment which has a passivation film comprised of chromium oxides,the gas supply unit is provided with a cleaning gas supply unit forremoving, by means of etching, deposits which stick to the interiorwalls of the reduced pressure chamber. Chlorine and fluorine gases aredesirable for use as cleaning gas. The reduced pressure chamber may beprovided with a heater capable of heating the chamber to about 350° C.

The reduced pressure device according to the present invention mayinclude one or a combination of two or more semiconductor manufacturingunits, gas cylinders, gas valves and pipes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a reduced pressure deviceillustrating one embodiment of the present invention;

FIG. 2 is a schematic representation illustrating a cylinder for usewith an oxidization method;

FIG. 3 shows the construction of an experimental apparatus for examiningthe degassing characteristics;

FIG. 4 is a graph showing the experimental results of the device of FIG.3;

FIG. 5 is a time chart showing an elevated temperature cycle;

FIG. 6 and FIGS. 7(a)-7(d) are graphs showing the distribution ofelements in a surface after oxidation; and

FIG. 8 is graph showing the relationship of the total leakage amount andthe concentration of impurities for a prior art device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a schematic representation of a reduced pressure deviceillustrating one embodiment of the present invention. In FIG. 1, 201 isa vacuum exhaust unite for example, a turbomolecular pump with anexhaust volume of 2000 l/min; 202 is a reduced pressure chamber which isconstructed of stainless steel (SUS316L) and on the inner wall of whicha passivation film is formed; and 204 is a gas supply system Byintroducing gas into the reduced pressure chamber 202 at a small flowrate, for example, 0.01˜100 cc/min, the reduced pressure chamber can bemaintained at a reduced pressure state, for example, 1×10⁻⁴ ˜1×10⁻¹Torr, with a fixed gas. Gas supply system 204 is therefore composed ofstainless steel pipes, joints, valves, mass flow controller, pressurereducing valves and ceramic filter, the detailed description of which isomitted.

Gas cylinder 205 supplies fixed gases to a gas supply device, includinggeneral gases such as Ar, He, H₂, O₂, etc., or corrosive gases such asHCl, Cl₂, BCl₃, etc., as desired. Although only one cylinder isillustrated in FIG. 1 for convenience, several cylinders 205 may be usedaccording to actual needs.

In addition, though only passivation film 203 of the inside surface ofreduced pressure chamber is illustrated in FIG. 1, the main parts of thegas supply system and the gas cylinder etc. are constructed of stainlesssteel (SUS316L, US304L and so on) and the passivation film of thepresent invention is formed on all interior surfaces which will be incontact with gas since these interior surfaces will therefore be exposedto corrosive gases.

In order to form a passivation film the surface of the stainless steelis first polished to a mirror finish. For example the method ofelectrolytic polishing may be used to cause the maximum heightdifference R_(MAX) between the concave parts and the convex parts of thesurface, i.e., the flatness of the surface, to be within 01˜1.0 μm. Thestainless steel surface is then heated at 400° C. in an atmosphere ofpure oxygen for about one hour. As a result oxidation film 203 is formedhaving a thickness of 110 Å.

The main component of the oxide is different in various areas of theoxidation films Iron oxide is the main component in the surface of theoxidation film and Cr oxide is the main component in the region near theinterface of the stainless steel and the oxidation film. The detailedresult of the oxidation composition will be described hereinbelow withTables 1, 2 and 3.

Mirror polishing of the surface of stainless steel may, for example beaccomplished with the technique of electrolytic polishing for interiorsurfaces of pipes. The technique of composite electrolytic polishing orthe like may be used for the interior surfaces of the chambers andcylinders. As for the method of oxidation, taking the oxidation of apipe as an example, after purging the interior surface of the pipe tosufficiently remove the moisture by using ultrapure Ar or He (moisturecontent less that 1 ppb), the interior surface is further purged at anelevated temperature of 150˜200° C., to release almost all of the H₂ Omolecules adsorbed in the interior surface. Electric current is thenapplied directly to the pipe to heat it to 400° C. to effect oxidationof its interior surface while passing pure oxygen with a moisturecontent of no more than 1 ppb through the pipe.

It is most important to form a passivation film in the gas cylinder forsuch a reduced pressure device because such a gas cylinder will containstock reactive gases for a long time. In order to keep the gas pure, apassivation film that is excellent in corrosion-resistance and does notocclude impure gases is nearly indispensable. The present invention has,for the first time, realized the formation of this kind of film. Themethod of oxidizing the cylinder is shown in FIG. 2.

For example, in the case of oxidizing the interior surface of gascylinder 301, to remove moisture, ultrapure Ar or He at room temperatureis introduced into the cylinder at a flow rate 10 l/min from gasintroducing pipe line 302 to purge the cylinder sufficiently. Dewpointhygrometer 305 disposed in purge line 304 is used to determine whethermoisture removal has been completed. After that, the whole cylinder isheated to 150˜400° C. by the use of electric heater 303 to releasenearly all of the H₂ O molecules adsorbed in the interior surfaces.Ultrapure oxygen is then introduced into the cylinder and the cylinderis heated to a given temperature (for example 400-600° C.) by using theelectric heater to oxidize its interior surface. The passivation filmformed in such a way is extremely stable and resistant against corrosivegases such as HCl, Cl₂, BCl₃, BF₃ and the cylinder having thispassivation film can contain the corrosive gases for a long time withoutbeing damaged.

In FIG. 4, the experimental results of the degassing characteristics ofthis passivation film are shown. The experiment is performed for pipes2m in length and 3/8 inch in diameter. The construction of theexperimental device is shown in FIGS. 3. That is, experimental samplesof the Ar gas flow through gas purifying apparatus 401 and are thenpassed through SUS pipe 402 at the flow rate of 1.2 l/min. The moisturecontent contained in the gas is measured by APIMS 403 (AtmosphericPressure Ionization Mass Spectrograph).

The results of the sample purges at room temperature are shown in FIG.4. The types of pipe used in the experiment include pipe (A) having itsinterior surface polished by the method of electrolytic polishing, pipe(B) which underwent passivation treatment with nitric acid after beingpolished by the method of electrolytic polishing, and pipe (C) which hasthe passivation film of the present invention. FIG. 4 shows theexperimental results with curves A, B and C respectively. Beforeperforming the experiment the pipes were placed for about one week in aclean room wherein the relative humidity was 50% and temperature was 20°C.

It is evident from curves A and B of FIG. 4 that a large quantity ofmoisture was detected from both pipe A which was treated by electrolyticpolishing and pipe B which underwent a passivation treatment with nitricacid after electrolytic polishing. After passing gas through the pipesfor one hour the respective moisture contents detected from pipe A andpipe B are 68 ppb and 36 ppb. The fact that even after passing gas fortwo hours the moisture contents are still high at, respectively, 41 ppband 27 pb for pipe A and pipe B shows that the moisture contents of pipeA and pipe B do not decrease easily. However, with pipe C, which had thepassivation film of the present invention, the moisture content droppedto 7 ppb after passing gas for 5 minutes and furthermore dropped to 3ppb, the background level, after 15 minutes. This confirms that pipe Chas an excellent adsorbed gas release characteristic.

Subseguently, pipe 402 was heated by the use of electric power frompower source 404 to change the temperature of the pipe according to thetemperature-time charts Table 1 shows the average values of the moisturecontent at temperatures ranging from the room temperature to 120° C.,120° C. to 200° C., and 200° C. to 300° C. From the results shown inTable 1 it is obvious that the amount of moisture released from thestainless steel pipe of the present invention is an order of magnitudeless than that released from other pipes. This means that the pipeaccording to the present invention adsorbs little moisture and releasesthe moisture easily so that this kind of pipe is most suitable for beingused to supply ultrapure gas. Moreover, the reduced pressure chamber(vacuum chamber) 202 (FIGS. 1) of the present invention can realize adegree of vacuum of 10⁻¹¹ ˜10⁻¹² Torr after baking, which is anexcellent characteristic for an ultrahigh vacuum device.

The oxidation film obtained by the oxidation of a surface of stainlesssteel will now be explained. Table 2 shows the relationships of filmthickness and refractive index versus oxidation temperature andoxidation time in the case of oxidizing SUS316L and SUS304L withultrapure oxygen. Table 3 lists the thickness and refractive index ofthe oxidation films formed on the surface of SUS316L at 400˜600° C. withthe oxidation time being 1˜9 hours. At an oxidation temperature ofbetween 400° C.-500° C., film thicknesses only depend on the temperatureof without correlating with the oxidation time. This suggest that theoxidation process of SUS is the process described by the model of Cabreaand Mott. Therefore, if the temperature is kept constant, the oxidationsfilm will grow to a given film thickness and a dense oxidation film ofuniform thickness and without pin holes can be formed.

On the other hand, because the process of oxygen diffusion in oxidationfilm will occur at the same time when the temperature rises to 550° C.or 600° C., the oxidation film will grow little by little along with theoxidation time.

FIG. 6 illustrates the element distribution in the surface of SUS316L,measured by ESCA, after oxidation at 500° C. for one hour. It is knownthat the Fe concentration is high in the vicinity of the surface and theCr concentration is high in the deep part. This fact shows that the Feoxide in the vicinity of the surface and the Cr oxide in the vicinity ofthe interface of the oxide film and SUS substrate form a two-layerstructure. This fact is confirmed by the energy analysis of the ESCAspectrum. The result shows that the chemical shift due to the formationof the Fe ocide in the vicinity of the surface can be observed and thatin the deep part this chemical shift does not occur, and moreover thatthe chemical shift due to the formation of the Cr oxide cannot beobserved except in the deep part. The reports about the formation ofsuch a two-layer structure through the oxidation of SUS have not beenconfirmed up to now. Although the mechanism and the reason for theexcellent corrosion-resistance and adsorbed gas releasing characteristicof the reduced pressure device of the present invention are not entirelyclear, it may be thought of as the result of the formation of this densetwo-layer film.

In addition, in order to form a, dense oxidation film it is important toremove the transmuted layer on the surface of the stainless steelproduced during its surface machining to make its surface flat. Althougha magnitude of R_(max) ranging from 0.1˜1.0 μm is used as surface finishin the present embodiment, from the experimental results it has beenknown that, if the maximum height difference of concave parts and convexparts within any circle 5 μm in radius is less than 1 μm, a good enoughpassivation film can be formed.

FIGS. 7(a)-7(d) show, corresponding to FIG. 6, the element distributionof the surface of a passivation film formed under changed oxidationconditions.

The oxidation conditions of FIGS. 7(a), (b), (c), (d) respectively, are(a) 400° C., 4 hours; (b) 500° C., 4 hours (c) 500° C, 4 hours; (d) 550°C., 9 hours. With the rise of the oxidation temperature and the increseof the oxidation time the rate of growth of chrome oxide film in thevicinity of the surface becomes high. At the condition of oxidationtemperature 550° C., oxidation time 9 hours, the quantity of chromeoxide film is more than that of Fe oxide film even in the vicinity ofthe surfaces. With the increase of chrome oxide film in the vicinity ofthe surface, the passivation film is more resistant against chemicalcorrosion. For example, a surface having this kind of passivation filmis not corroded at all by gases of the chlorine system. As asemiconductor device, it is the routine duty of the reaction chamber todeposit several layers of insulator films such as Si single crystal,SiO₂, Si₃ N₄, and AlN, and films of metals such as Al, Al--Si,Al--Cu--Si, W, Mo, Ta, Ti, Tin, and Cu. Generally, these films areformed by the methods of CVD (Chemical Vapor Deposition) and Sputter.With prior art devices, even when those films were deposited onsemiconductor wafers, large amounts of deposits adhered to the innerwall of the reaction chamber during the formation of these films.Because these extraneous materials will become the sources of dust andresult in pattern defects in films, for a CVD device it is necessaryalmost every day to open the reaction chamber and to remove depositsadhering to the inner wall of the chamber, which leads to an extremelylow working ratio. However, by using the reaction chamber having apassivation film of the present invention on its interior surface,deposits adhering to the inner wall of reaction chambers can be removedby etching by elevating the temperature of the reaction chamber to200˜350° C. at regular intervals and passing cleaning gas through thereaction chamber such as chlorine gases, for example Cl₂ and HCl gases,or fluorine gases. Due to the extreme flatness of the interior surfaceof the chamber, and of the passivation film which is excellent in bothchemical and mechanical properties which is formed on the interiorsurface of the reaction chamber, the adhesive strength of the depositsadhering to the interior walls of the reaction chamber is very weak andthe removal of deposits by etching is therefore exceedingly effective.

Thus a semiconductor manufacturing device possessing the properties ofself-cleaning and self-maintenance has been realized for the first timeaccording to the present invention. With the development of thesemiconductor manufacturing device possessing the characteristics ofself-cleaning and self-maintenance, the full automation of semiconductormanufacturing lines also become possible for the first time.

As listed in Table 2 and 3, if the surfaces of stainless steel areoxidized in very pure oxygen at a temperature higher than 400° C. toform a passivation film, after mirror polishing of the surfaces ofstainless steel and after washing and drying the surfaces sufficiently,film thicknesses are usually larger than 100 Å.

Although the above-stated description concerned the device shown in FIG.1 as an embodiment of the present invention, the present invention isalso applicable to an ultrahigh vacuum experimental device, RIE device,sputter device, CVD device or the like. As shown in FIG. 1, allcomponents including the gas supply system, gas cylinder and so oncomprise a reduced pressure device. Therefore it is self evident thatgas cylinders and gas valves and pipes, on the interior surfaces ofwhich passivation films of the present invention are formed, areincluded in the reduced pressure device of the present invention. Thesame is also true of vacuum pumps. Further it is evident that themechanical parts disposed in reduced pressure chamber 202, for example amechanism for transporting wafers, are also included in the reducedpressure device of the present invention if passivation films are usedon these parts.

Additionally, though all of the exposed surfaces in the interior of thedevice are almost completely coated with the oxidation films in thepresent embodiment, it is also possible to form the oxidation films onlyon the surface regions that can make the most of the formation ofoxidation films such as large interior surface areas, for example, theinterior surfaces of a reduced pressure chamber.

And moreover, although the stainless steels of SUS304L, SUS316L are usedin the above embodiment, the stainless steels of the Fe--Cr system, andFe--Cr--Ni system can also be used. Further, as for the structures ofstainless steels, the ferrite system, martensite system, and austenitesystem can all be used.

According to the present invention, the quantity of impurities due togases released from interior surfaces of a reduced pressure device canbe reduced. Furthermore, an ultrahigh vacuum and ultra-clean reducedpressure device having excellent corrosion-resistance can be realized byforming oxidation films containing no pin holes on the interior surfacesof a reduced pressure device constructed of stainless steel.

With the realization of a reduced pressure device having passivationfilms that are excellent in corrosion resistance and which almost do notrelease gases from its interior surfaces, ultrahigh vacuum andultra-clean reduced pressure processes have become possible.

In addition to the above-mentioned effects, self-cleaning andself-maintenance of a reduced pressure device have also become possible.

While this invention has been described as having a preferred design, itwill be understood that it is capable of further modification. Thisapplication is therefore intended to cover any variations, uses oradaptations of the invention following the general principles thereofand including such departures from the present disclosure as come withinknown or customary practice in the art to which this invention pertainsand which fall within the limits of the appended claims.

                  TABLE 1    ______________________________________    amount moisture released from the sample pipe    Temp. (°C.)               Room temp.     120    200    Sample     ˜120     ˜200                                     ˜300    ______________________________________    electrolytic               420            600    860    polished    treated with               750            630    990    nitric acid    this invention               25             70     100    ______________________________________     unit (ppb)

                  TABLE 2    ______________________________________    thickness of passivation film               SUS316L     SUS304L    oxidation            oxidation                     film            film    temperature            time     thickness                              refractive                                     thickness                                            refractive    (°C.)            (hr)     (Å)  index  (Å)                                            index    ______________________________________    400     1        114.0    2.71   78.8   3.26            4        110.9    2.87   74.2   3.41    500     1        125.7    2.93   95.8   3.60            2        126.1    2.91   95.2   3.50            4        126.8    2.96   91.3   3.81    550     1        130.9    3.02   102.9  3.56            4        141.8    3.13   110.9  3.76    ______________________________________

                  TABLE 3    ______________________________________    passivation film of SUS316L    oxidation oxidation    film    temperature              time         thickness                                    refractive    (°C.)              (hr)         (Å)  index    ______________________________________    400       1            114      2.71              4            110      2.87    500       1            125      2.93              2            126      2.91              4            126      2.96              9            124      2.90    550       1            130      3.02              4            141      3.13              9            164      2.98    600       1            156      3.03              4            180      3.36    ______________________________________

I claim:
 1. A method for forming a passivation film on a stainless steelsurface, the method comprising the step of oxidizing the stainless steelsurface to form a film with a thickness of more than 100 Å by heatingthe stainless steel to a temperature in the range of 400° C.-550° C.,after purging moisture, using an inert gas with a moisture content ofless than 1 ppb, from said surface.
 2. A method for forming apassivation film on a stainless steel surface, the method comprising thestep of oxidizing the stainless steel to form a film of more than 130 Åin thickness by heating the stainless steel above 550° C. for at leastnine hours, after purging moisture, using an inert gas with a moisturecontent of less than 1 ppb, from said surface.